Field of the Invention
The present invention relates to a pattern inspection apparatus and a pattern inspection method. For example, it relates to an inspection apparatus that inspects a pattern by comparing an optical image of a pattern image, which is obtained by irradiating illumination light, with a reference image, which is generated from configuration data etc., and relates to a method therefor.
Description of Related Art
In recent years, with the advance of high integration and large capacity of a large scale integrated circuit (LSI), the line width (critical dimension) required for a circuit of a semiconductor element is becoming narrower and narrower. Such semiconductor elements are manufactured by exposing (transferring) a pattern onto a wafer to form a circuit by means of a reduced projection exposure apparatus, which is known as a stepper, by using an original or “master” pattern (also called a mask or a reticle, and will be generically referred to as a mask hereinafter) with a circuit pattern formed thereon. Therefore, in manufacturing a mask for transferring such a fine circuit pattern onto a wafer, a pattern writing apparatus using electron beams capable of writing or “drawing” fine circuit patterns needs to be employed. Pattern circuits may be written directly on a wafer by the pattern writing apparatus. Alternatively, a laser beam writing apparatus which uses laser beams in place of electron beams for writing a pattern is under development.
Since the LSI manufacturing requires a tremendous amount of manufacturing cost, it is crucial to improve its yield. However, as typified by a 1 gigabit DRAM (Dynamic Random Access Memory), the scale of a pattern configuring an LSI has been changing from on the order of submicron to on the order of nanometer. One of major factors that decrease the yield of the LSI manufacturing is a pattern defect of a mask used when exposing (transferring) a fine pattern onto a semiconductor wafer by the photolithography technology. In recent years, with miniaturization of an LSI pattern formed on a semiconductor wafer, dimension to be detected as a pattern defect have become extremely small. Thus, a pattern inspection apparatus for inspecting a defect of a transfer mask used in manufacturing LSI needs to be highly accurate.
Meanwhile, with development of multimedia technology, the size of LCD (Liquid Crystal Display) substrate is becoming larger, e.g., 500 mm×600 mm or greater, and the size of a pattern such as a TFT (Thin Film Transistor) or the like formed on the liquid crystal substrate is becoming finer. Therefore, it is increasingly required that an extremely small defect of a pattern should be inspected in a large range. For this reason, development of a pattern inspection apparatus that efficiently and short-timely inspects a defect of a photomask used when manufacturing patterns of a large area LCD and a large-area LCD is urgently required.
As an inspection method, there is known a method of comparing an optical image of a pattern formed on a target object or “sample”, such as a lithography mask, imaged at a predetermined magnification by using a magnifying optical system with design data or an optical image obtained by imaging the same pattern on the target object. For example, the following is known as pattern inspection methods: die-to-die inspection method that compares data of optical images of identical patterns at different positions on the same mask, and die-to-database inspection method that inputs, into the inspection apparatus, writing data (design pattern data) which is generated by converting pattern-designed CAD data to a writing apparatus specific format for input when writing a pattern on the mask, generates design image data (reference image) based on the input writing data, and compares the generated design image data with an optical image (serving as measurement data) obtained by imaging the pattern. According to the inspection method of the inspection apparatus, a target object is placed on the stage so that a light flux may scan the object by the movement of the stage in order to perform an inspection. Specifically, the target object is irradiated with a light flux from the light source and the illumination optical system. Light transmitted through the target object or reflected therefrom is focused on a sensor through the optical system. An image captured by the sensor is transmitted as measurement data to the comparison circuit. In the comparison circuit, after position alignment of the images, measurement data and reference data are compared with each other in accordance with an appropriate algorithm. If there is no matching between the measurement data and the reference data, it is judged that a pattern defect exists.
While with the advance in miniaturization of patterns, it continues to use the photolithography technology that forms a circuit by exposing and transferring a pattern onto a wafer by means of a reduced projection exposure device using a mask serving as a pattern original. Then, in order to increase a manufacturing yield of wafers, the allowable accuracy range (margin) with respect to a pattern defect of a mask and the allowable variation range (margin) with respect to process conditions in exposing and transferring are becoming tight. Until now, since the accuracy of dimension of a mask pattern has been kept high by having a tight range of tolerance values with respect to a shape defect, even when a variation margin of process conditions is somewhat wide, the width has been compensated for by the tightness of the accuracy of mask pattern dimension. However, recently, it is increasingly required for quality of a mask to be inspected to have uniformity of pattern position accuracy and pattern dimension errors on the whole surface of the mask in addition to having the accuracy that figures composing a pattern are formed exactly in accordance with the design respectively. Conventionally, the quality control of the uniformity of a mask surface has been performed by grasping errors of extension and contraction, parallel shifts, and local variation which are measured based on positions of cross marks arranged at suitable intervals in the mask pattern, by using a dedicated measuring device, in order to measure a variation amount of each point coordinates which should be an ideal grid.
Mask defect inspection usually performed is designed to detect a pattern shape defect, and thereby, a defect dimension to be detected becomes fine in accordance with the miniaturization of mask patterns. Accordingly, the detection needs to be performed by using a detection optical system of high magnification, and the inspection time required for inspecting a defect of the whole mask surface has come up to several hours, for example. Then, during such several hours of mask defect inspection, there occurs a problem in that a pattern position and a pattern dimension obtained based on a measured result vary in the mask surface by a thermal expansion of the mask to be inspected, which is generated from the storage of energy of inspection light irradiated on the mask, by a measurement error of the stage position measuring system, which is generated from a change of air current inside the inspection apparatus or various heat sources in the apparatus, and so on. Consequently, the uniformity of pattern position accuracy and the uniformity of pattern dimension errors in the whole surface of the mask are degraded, and the level of such degradation of the uniformity has become too high to ignore for the quality precision of the mask. Thus, to inspect the uniformity is an issue to be solved for assuring the reliability of the inspection apparatus and the mask to be inspected.
A pattern dimension error can be calculated, for example, by detecting edges at both the ends of a pattern from a measured optical image, obtaining the distance between the edge pair in order to measure a line width dimension of the measured optical image, detecting edges at both the ends of a pattern from a reference data image, and obtaining the distance between the edge pair in order to measure a line width dimension of the reference data image, so that the pattern dimension error is calculated by obtaining a difference between the line width dimensions (refer to, e.g., Japanese Patent Application Laid-open (JP-A) No. 3824542). However, the dimension error calculated herein includes, as described above, an error due to the inspection apparatus, such as a thermal expansion of the inspection mask, which is generated from the storage of energy of inspection light irradiated on the mask, and a measurement error of the stage position measuring system, which is generated from a change of air current inside the inspection apparatus or various heat sources in the apparatus. Therefore, there is a problem that it cannot be known whether the dimension error belongs to a pattern formed on the mask or the dimension error is due to the inspection apparatus.